[SDK] Add credentials option to OTLP gRPC client (#3402)

This allows passing custom `ChannelCredentials` when creating gRPC channels, to support authentication protocols that require short-lived tokens.

Author: DouglasHeriot

exporters/otlp/include/opentelemetry/exporters/otlp/otlp_grpc_client_options.h

@@ -3,6 +3,8 @@
 
 #pragma once
 
+#include <grpcpp/security/credentials.h>
+
 #include "opentelemetry/exporters/otlp/otlp_environment.h"
 #include "opentelemetry/version.h"
 
@@ -43,6 +45,9 @@ struct OtlpGrpcClientOptions
   std::string ssl_client_cert_string;
 #endif
 
+  /** Use custom ChannelCredentials, instead of the SSL options above. */
+  std::shared_ptr<grpc::ChannelCredentials> credentials;
+
   /** Export timeout. */
   std::chrono::system_clock::duration timeout;
 

exporters/otlp/src/otlp_grpc_client.cc

@@ -392,7 +392,11 @@ std::shared_ptr<grpc::Channel> OtlpGrpcClient::MakeChannel(const OtlpGrpcClientO
   }
 #endif  // ENABLE_OTLP_RETRY_PREVIEW
 
-  if (options.use_ssl_credentials)
+  if (options.credentials)
+  {
+    channel = grpc::CreateCustomChannel(grpc_target, options.credentials, grpc_arguments);
+  }
+  else if (options.use_ssl_credentials)
   {
     grpc::SslCredentialsOptions ssl_opts;
     ssl_opts.pem_root_certs = GetFileContentsOrInMemoryContents(